Data processing apparatus such as a computer or telecommunications apparatus such as a modem or an exchanger includes a data transmission apparatus for exchanging information with the outside world. The data transmission apparatus includes a plurality of I/F sections each for transmitting data outward.
In general, the I/F section consumes a larger amount of power as compared with other circuits in the apparatus. This is because the I/F section transmits the data outward. Especially, the consumption is remarkable in the case of outputting electromagnetic waves or laser light outward or in the case where many I/F sections are provided.
For this reason, in portable devices it is especially necessary to reduce power consumption because a data transmission apparatus includes a plurality of power source units for supplying the respective I/F sections with electric power. It is widely adopted that the power source units control the supply of the electric power to the I/F sections such that the power source units stop supplying the electric power to the I/F sections that have no outside inputting/outputting of signals. When the power source units delicately control the supply of the electric power to the I/F sections, the data transmission apparatus can reduce power consumption.
However, the more complicated data transmission apparatus causes an increase of the number of components, an increase of the manufactures' cost, and an increase of the size of the apparatus. This is because the data transmission apparatus needs plural power source units whose number is equal to the number of the I/F sections.
Further, an inexpediency may arise, depending on what kind of signals are inputted and outputted between the I/F sections and a control unit for outputting data to the I/F sections: when data is supplied to the I/F section while the power source unit stops supplying the electric power to the I/F section, an internal circuit of the control unit or the I/F section is likely to break down.
Still further, the problem arises that reduction of the power consumption is not possible for an apparatus in which a power source unit does not carry out the above power control with respect to an I/F section.